1. Field of the Invention
The present invention relates to an electronic instrument such as a camera receiving electric power from a main battery and a backup device, and more particularly to an electronic instrument which includes electronically operating devices such as a microcomputer, a memory device or the like.
2. Description of the Prior Art
In recent years an increasing number of electronic instruments are provided with a microcomputer in order to control various devices thereof with high precision. With such an electronic instrument, varied control data are stored in the memory of the microcomputer. It is common practice to use a battery as an electric power source for supplying power to the microcomputer in a camera or other portable instrument, and the battery must be replaced when its capacity is exhausted through use. When the battery is taken out of the camera for replacement, data stored in the memory which normally are of the volatile nature will be cancelled and disappear. Thus, some backup means such as a backup battery is provided to supply power to the microcomputer when the battery normally acting as the power source, or a main battery, is replaced.
However, if the capacity of the backup battery has been exhausted by the time the main battery is taken out for replacement and power should be supplied to the microcomputer by the backup batter, the output voltage of the backup battery does not reach a normal operating voltage of the microcomputer and does not guarantee proper maintenance of the data stored in the memory. When in such a case a new main battery is loaded into the camera and the microcomputer resumes its operation, the ill-maintained information in the memory will be used which results in malfunctioning of the varied devices acting under control of the microcomputer.
Furthermore, consumption of the backup battery will progress to an undesirable extent when a new main battery is not loaded for a long time or its loading is forgotten after an old main battery is taken out for replacement. And when the voltage of the backup battery drops below a critical voltage for memory operation, the contents of the memory such as a random access memory (RAM) will be erased. Where the electronic instrument includes an indicating device for indicating the results of calculations or control states of the microcomputer, the indicating device may be in operation although the data in the RAM have been erased. This is because the RAM has a much narrower range of operating voltage than the indicating device, and will mislead the user to believe from the indications of the indicating device that the electronic instrument is in condition for proper operation. When the electronic instrument is operated in that state, inconveniences such as its malfunctioning or breakdown will be encountered.
In another known arrangement such as disclosed in Japanese utility model applications laid open for public inspection under Nos. 60-31669 and 60-31670, the microcomputer is reset when a main battery is loaded and when the capacity of the backup battery reduces below a predetermined value. With the above arrangement, the microcomputer is reset when a new main battery is loaded, and all the data will be erased from the memory even if properly preserved by the backup battery having a sufficient capacity. In the case of camera, for example, it is then necessary to set film sensitivity, the number of exposed frames of film and other data all over again.
Moreover, even if the main battery alone is sufficient to supply power to the microcomputer, the microcomputer is reset when the capacity of the backup battery drops below the predetermined value. Therefore all the devices controlled by the microcomputer will stop operating.
In the case of an electronic instrument wherein at least a small-load circuit such as the memory receives power supply from the backup battery only when the output voltage of the main battery has reduced or when the main battery is replaced with a new one, generally the backup battery is already mounted in the instrument but the main battery is not loaded at the time of shipment from a factory. The main battery is loaded by the user for starting operation of the devices in the instrument or camera. With this type of instrument, the power supply from the backup battery to the memory is allowed in an initial state, namely from the time of shipment from the factory without the main battery being loaded in the instrument.
In other words, the memory receives constant power supply from the backup battery until the user purchases the instrument and loads it with a main battery for the first time. Generally, power consumed by the memory is very small compared with power consumed by a great-load circuit which receives power supply from the main batter, but the capacity of the backup battery will be reduced with lapse of a long time after the shipment from the factory.